1. Field of the Invention
The invention relates to a workpiece end detecting apparatus for a sewing machine. More particularly, the invention relates to an apparatus for detecting not only the workpiece end but also the differences in workpiece thickness.
2. Description of the Prior Art
Conventional industrial lockstitch sewing machines may contain a combination photo sensor and light projector located near the needle entry. This detecting device is effective to sense the workpiece end or the difference in thickness of a workpiece. Such detecting devices are thus able to detect overlapping conditions, for example, when stitching a pocket to a shirt or stitching a label to a workpiece. In some cases, a reflecting type sensor is used in lieu of the above sensor. Non-contact and light-projection type sensors have thus been commonly used, projecting a light from the upper side of the workpiece, the light penetrating through the workpiece and reaching the photo sensor located under the throat plate. The photo sensor detects the workpiece end or the difference of workpiece thickness by sensing the variation of light penetration. Such light-projection type detectors are well-known and widely used because they are simple to operate, are inexpensive and require no contact with the surface of the workpiece.
However, the conventional types of non-contact sensors have drawbacks, including the following:
(1) light-projection type detectors are not applicable where the workpiece is opaque, such as with leather or very thick workpieces; PA0 (2) where the detector is located far from the presser foot, it is difficult to detect correctly because the workpiece is fluttered by the dog-feeding motion, and in such cases some auxiliary pressure is ordinarily needed to stop the fluttering, thereby reducing the advantages of non-contact, photo-sensor type detectors; PA0 (3) to change the location of photo sensor, the throat plate ordinarily has to be changed; PA0 (4) electric wires from the photo sensor are problematic and obstructive when changing the looper thread; PA0 (5) dust or dirt may accumulate on the optical parts of the prior known detectors, lowering the sensitivity of the photo sensor and thus requiring frequent cleaning by the operator; PA0 (6) unintentional obstruction by operator's finger or hand may cause the photo sensor to malfunction.
It is therefore an object of the invention to provide a contact type workpiece detector which eliminates the drawbacks of conventional detector systems, is simply constructed and which may be used for any kind of workpiece.